VFD as soft starter
Welcome to the next article in our motor starting series. This time we will talk about VFD as a soft starter. Our first post focused on direct on-line motor start as the cheapest and simplest method. The second article provided a brief overview of common motor starting methods, highlighting their strengths and drawbacks. Finally it is time to look at motor starting method with highest quality: VFD soft start.
Variable frequency drive is usually utilized for continuous duty to control the speed or torque of electric motor. Sometimes, VFDs are also used as soft starters (our subject today). We shall not mistake them with cheap soft starters varying the voltage only (with fix frequency). The purpose of VFD soft starter is to smoothly ramp up the speed of the motor up to the nominal speed and transfer it to the grid.
Although the soft start might seem much simpler than continuous operation there are some specific topics to be considered. The VFD soft starter is often underestimated leading to unnecessary issues on site. But that is more related to VFD dimensioning for a soft starting duty. We will report on that topic in a dedicated article.
VFD soft start
How does a soft start with VFD look like? The VFD supplies the motor with voltage and frequency that are both ramped up. Normally the ratio between VFD output voltage and output frequency is kept constant (V/f = const) corresponding to the nominal flux of the machine. As the motor approaches the nominal speed, the synchronization unit kicks in. It has a clear task – synchronize the machine voltage, frequency and angle with those of the supplying grid. Once these quantities are within the tolerance a synchronization can take place. After a successful synchronization the VFD can either be switched off or soft start the next motor.
VFD soft start is the method to provide the highest quality of starting an electric motor – from grid perspective as well as motor/load perspective. Let’s repeat them once again.
Advantages of VFD soft starting
Advantages of a VFD soft start had already been mentioned in our last post. Therefore, we just quickly refresh what we have learnt.
(i) Smooth starting current
The motor starting current remains below the nominal current (FLC) during entire start-up. No other starting method provides such low and smooth motor current without compromising the motor performance.
Unlike other starting methods listed in [2], the VFD starts the motor in a frequency controlled manner. The inverter output frequency ramps up from zero up to the grid frequency. Synchronous machine remains in synchronism during entire start-up. Asynchronous machine operates with small slip within the stable area of torque-speed curve.
(ii) No significant voltage drop in the grid
As the starting current is low there is no reason for a significant voltage drop. In fact, the maximum voltage drop during start-up does not exceed the normal voltage drop at full load. As such, there is no issue for other consumers connected to the same node. VFD soft starting causes the least voltage drop from all solutions described in [2].
(iii) Full motor torque straight from standstilll
VFD soft starter can provide rated motor torque from zero speed. Of course, the torque is adjustable and can be reduced as well. IN a speed controlled mode the torque is set by the speed controller depending on the speed ramp. VFD soft start is the only method that can provide full motor torque at any speed.
(iv) Decoupling from grid voltage fluctuation
In context of previous point it is worth to mention that VFD can and usually also does maintain nominal flux in the machine during start-up even if there is a grid undervoltage. Therefore, the machine can provide its rated torque also when the grid voltage is below its nominal value. As a reminder, in direct on-line start the motor torque decreases with the square of the supply voltage.
(v) No severe torque pulsations
It is not just about the mean value of starting torque. We have shown in [1] that direct on-line start creates large torque pulsations right after starting from standstill. The word ‘large’ is definitely not exaggerated as the torque peaks are multiples of the rated torque. Unlike across the line starting, a VFD soft start does not generate any considerable torque oscillations. Instead, there is just a normal torque ripple with magnitude of 5-10% of rated torque. Especially the mechanical system appreciates such behavior and will reward you with an extended lifetime and much less maintenance work.
(vi) No limitation in number of starts
Direct on-line start represents a severe thermal stress for the motor. Starting current often reaches up to 500 to 700% of nominal current and the heat losses are proportional to the square of the current. The heavier the start (large inertia and/or high load torque) the bigger the thermal challenge. No surprise that after 2-3 starts the machine needs to rest for a decent period of time and cool down.
There is no such limitation in case of VFD soft start. As the current normally does not exceed rated current there is no source of considerable heat losses. Motor can be started via VFD soft starter as often as required without compromising machine lifetime.
(vii) Freedom to optimize the motor design
Motor design for a direct on-line start has several constraints. The starting current shall be kept within certain acceptable range. At the same time, the machine shall provide sufficient starting torque (also at undervoltage). Finally, the machine shall have as high efficiency as possible. Fulfilling all these requirements is not easy. Actually, a compromise must be made.
VFD soft start leaves much more freedom to optimize the machine for the normal operation after the start-up. You don’t need to downgrade the machine efficiency just to get enough starting torque.
(viii) Improved efficiency
VFD for continuous variable speed control are known for considerable energy savings. Also a VFD soft starter can save energy and improve efficiency. How? As explained above, VFD soft starting allows more freedom to optimize the motor design, particularly the motor efficiency. The motor designer is not bound to certain minimum rotor resistance that would give him sufficient starting torque. Instead, the motor can be designed with better effficiency. This fact is surely not to underestimate as major portion of total lifecycle cost of an electric motor is the cost of electric energy.
At this place we would like to remind you that energy cost of an electric motor typically make over 90% of lifecycle cost unlike the purchase price that makes only approx. 5% of lifecycle cost. Any small improvement in motor efficiency results in a significant cost benefit accumulated over the lifetime.
(ix) Inherent motor protection during soft start
VFD provides all necessary protections functions to protect the machine during a soft start. These are the same functions as used in VFD for continuous duty – such as overcurrent, over- and undervoltage, overflux, motor stalling, ground fault protection etc. Some restrictions apply in case of an adjusting transformer between inverter and motor.
Drawbacks of VFD soft start?
The list of advantages of VFD soft starter looks really great. So hand on our heart: are there also some disadvantages?
The main disadvantage from user’s perspective is most likely the price tag of this solution. Honestly speaking, VFD for soft starting duty is not much cheaper than a VFD for continuous duty. The power electronics has very short thermal time constant. Therefore, the VFD typically needs to be dimensioned for peak power although it has just a short duration. Sometimes transient thermal calculation can be used to optimize the solution a bit. VFD transformers, if required, benefit more from the short duty cycle.
However, the cost disadvantage becomes less of an issue if the VFD can be utilized in a smart way. For instance, one VFD can soft start several motors, one after another. The more motors for a common VFD the better it looks commercially.
Even better solution can be realized in case the VFD soft starts and synchronizes one or more motors while it is used as a continuous duty VFD for the last motor. This is a non-fiction scenario realized several times, e.g. in pump stations or in a mining industry (soft start and synchronize one or more ball mills and then continuously control the speed of a SAG mill).
Another disadvantage might be increased space requirements (footprint) compared to more simple solutions, such as starting reactor or auto-transformer [2].
Transfer from VFD to the grid
There are two principles to synchronize and transfer the motor from the VFD supply to the grid. Main difference is the sequence of the switches.
1. Open before close
The first method is called to as ‘open before close’ or ‘break before make’. Reference is made to the sequence how the switches are operated. The motor is first disconnected from the VFD and afterwards the motor is connected to the grid.
Advantage of this method is a good protection of the VFD and no need for additional hardware.
Disadvantages are torque bump during the transfer from VFD to grid, certain transient current and also need to properly interlock the switches. Speed drop during such transfer might be an issue for applications with very low inertia.
Generally, speed drop can be minimized or even fully compensated.
2. Close before open
Second method is, surprisingly, called ‘close before open’ or ‘make before break’. In this case the motor is accelerated to the nominal speed and when the conditions for synchronization are fulfilled, the motor is first connected to the grid and just after that the VFD gets disconnected.
Advantages of this solution are “bumpless” synchronization (no torque bump), absence of transient current and generally a smooth transfer from VFD supply to the grid.
Disadvantage is the need for additional reactor and voltage drop across this reactor.
Soft starting of a large motor
Many times the motor can be started with reduced load (e.g. by starting up decoupled from the load or by opening certain valves). Therefore, VFD soft starter does not need to be rated for full motor power. Despite that, in case the motor rating is significantly larger than the VFD capacity, a special care must be taken. The system designer shall ensure proper protection of the VFD during start-up and also make sure that VFD can supply required reactive power (valid for asynchronous machines). Finally, the parameterization of the control software might need some adjustments as well.
Multi-motor application
VFD soft start is the best motor starting method from performance point of view. Though, it is usually also the most expensive one. Cost of the solution can be considerably reduced when using one VFD to soft start multiple motors one-by-one.
VFD topology for soft starting
Exact same VFD topologies as for continuous speed control are used for soft starting purpose. In lower power range voltage source inverters (VSI) are common. In very high power range, a domain of synchronous machines, the LCI is still the major choice due to its simplicity and cost competitiveness.
Depending on motor voltage and VFD topology the inverter either directly supplies the motor or there is a transformer in between.
Summary
VFD soft start is the most advanced method of starting an AC motor. Main benefits are absence of high starting current and torque transients, consequently a very small grid (bus) voltage drop, ability to provide full motor torque from standstill, adjustable speed ramps and potentially higher motor efficiency.
VFD soft starter is associated with higher investment cost. However, this extra cost might be compensated or even outperformed by advantages such as less maintenance of mechanical components or higher motor efficiency.
The transfer of motor from VFD to grid operation can be realized as open before close or close before open transition.
References
[1] Motor starting methods: Direct on-line start, https://mb-drive-services.com/direct-on-line-start/
[2] Motor starting methods: Overview, https://mb-drive-services.com/motor-starting-methods/
[3] Motor start with a VFD, https://mb-drive-services.com/vfd-motor-start/
[4] Medium voltage AC drives, https://new.abb.com/drives/medium-voltage-ac-drives
[5] Motors & Generators made by ABB, https://new.abb.com/motors-generators
